GDM is diabetes that develops during pregnancy. After delivery, the woman may remain a diabetic, develop diabetes in the future, or never again experience any hyperglycemia. Risk assessment and screening for diabetes is routine in prenatal care because of the pregnancy complications and mortality associated with GDM. For many years, GDM was defined as any degree of glucose intolerance first recognized during pregnancy, regardless of whether the condition existed before or after pregnancy. More recently, women with diabetes in the first trimester are typically classified as having type 2 diabetes. On the other hand, GDM is mainly defined as diabetes diagnosed in the second or third trimester of pregnancy that is not clearly overt diabetes.

Testing and diagnosis of GDM: The testing and diagnosis of GDM is typically accomplished during a woman's first prenatal visit in those individuals with risk factors. In pregnant women not previously known to have diabetes, testing for GDM is usually done at 24-28 weeks of gestation. In addition, women with GDM are typically screened for persistent diabetes at 4-12 weeks postpartum. Moreover, women with a history of GDM should have lifelong screening for the development of diabetes or prediabetes at least every 3 years.

In 2013, the National Institutes of Health (NIH) convened a consensus development conference on diagnosing GDM. The 15-member panel had representatives from obstetrics/gynecology, maternal-fetal medicine, pediatrics, diabetes research, biostatistics, and other related fields to consider diagnostic criteria. The panel recommended the two-step approach of screening with a 1 hour, 50 gram glucose load test (GLT), followed by a 3 hour, 100 gram oral glucose tolerance test (OGTT) for those who screen positive. This strategy is commonly used in the United States.